A semiconductor layer can have conductivity thereof controlled and is usable as an active layer of a thin film transistor (TFT). A TFT is provided in, for example, an active matrix type liquid crystal display device, and such a liquid crystal display device is used as a display of TVs, computers, mobile terminals and the like.
A semiconductor layer is formed on a support substrate together with an insulating film, electrodes, wiring lines and the like by a technique similar to a technique used for producing a semiconductor integrated circuit such as a thin film formation technique, photolithography or the like. For example, in a TFT of a top-gate structure, a semiconductor layer is provided on an insulating surface; and on the semiconductor layer, a gate electrode is provided with a gate insulating film interposed therebetween.
It is known that in the TFT of a top-gate structure, the gate insulating film is thinner on an edge of the semiconductor layer than on the other regions of the semiconductor layer and that the distance between the edge of the semiconductor layer and the gate electrode is shorter than the distance between the other regions of the semiconductor layer and the gate electrode (see Patent Document 1). The tendency that the thickness of the gate insulating film is non-uniform as described above is more conspicuous as the gate insulating film is thinner.
Patent Document 1 describes that when the thickness of the gate insulating film is non-uniform, a strong electric field is formed at an edge of the semiconductor layer by the fringe effect and so a side current is generated, and as a result, the characteristics of the TFT as a switching device are deteriorated. As the distance between the edge of the semiconductor layer and the gate electrode is shorter, the absolute withstand voltage of the TFT is decreased due to the concentration of the electric field at the edge of the wiring lines of the semiconductor device. This is likely to cause a dielectric breakdown. Under such a situation, Patent Document 1 discloses a structure for suppressing a change in the thickness of the insulating film on the edge of the semiconductor film.
FIG. 8 shows a TFT 500 disclosed in Patent Document 1. The TFT 500 includes a semiconductor layer 600, a gate insulating film 510 for covering the semiconductor layer 600, a gate electrode 520 provided on the semiconductor layer 600 with the gate insulating film 510 interposed therebetween, and a source electrode 530 and a drain electrode 540 which are in contact with the semiconductor layer 600. The semiconductor layer 600 is tapered at an edge thereof, which suppresses a change in the thickness of the gate insulating film 510 and thus suppresses the deterioration in the characteristics of the TFT 500 as a switching device.
Patent Document 1 describes a method for forming the semiconductor layer 600. Hereinafter, with reference to FIG. 9, the method for forming the semiconductor layer 600 will be described.
First, as shown in FIG. 9(a), a semiconductor film S is deposited, and then a photoresist layer R is formed on the semiconductor film S.
Next, the semiconductor film S is etched using the photoresist layer R as a mask. As shown in FIG. 9(b), a portion of the semiconductor film S which is not covered with the photoresist layer R is removed by the etching, thereby forming an island-like semiconductor layer. By the etching, a part of the photoresist layer R is also removed from a side face thereof. When the part of the photoresist layer R is removed from the side face thereof, a portion of the island-like semiconductor layer which is exposed by the removal of the photoresist layer R is gradually etched from a top surface toward a bottom surface thereof. Therefore, as shown in FIG. 9(c), the semiconductor layer 600 obtained as a result of the etching has a tapered edge. Then, the photoresist layer R is removed, thereby forming the tapered semiconductor layer 600.
Patent Document 1 also discloses another TFT having a step-like semiconductor layer. FIG. 10 shows a TFT 700 disclosed in Patent Document 1. The TFT 700 includes a semiconductor layer 800, a gate insulating film 710 for covering the semiconductor layer 800, a gate electrode 720 provided on the semiconductor layer 800 with the gate insulating film 710 interposed therebetween, and a source electrode 730 and a drain electrode 740 which are in contact with the semiconductor layer 800. The semiconductor layer 800 is stepped at an edge thereof, which suppresses a change in the thickness of the gate insulating film 710 and thus suppresses the deterioration in the characteristics of the TFT 700 as a switching device.
Patent Document 1 describes a method for forming the semiconductor layer 800. Hereinafter, with reference to FIG. 11, the method for forming the semiconductor layer 800 will be described.
First, as shown in FIG. 11(a), a semiconductor film S is deposited, and then a photoresist layer R is formed on the semiconductor film S.
Next, the semiconductor film S is dry-etched using the photoresist layer R as a mask. As shown in FIG. 11(b), a portion of the semiconductor film S which is not covered with the photoresist layer R is removed by the etching, thereby forming an island-like semiconductor layer S′. The etching is performed using a gas which removes the semiconductor film S but does not remove the photoresist layer R.
Next, as shown in FIG. 11(c), the photoresist layer R is ached to remove a part of the photoresist layer R. As a result, a part of the island-like semiconductor layer S′ is exposed. The aching is performed after the gas in the device is replaced with a gas which removes the photoresist layer R but does not remove the island-like semiconductor layer S′.
Next, as shown in FIG. 11(d), the island-like semiconductor layer S′ is etched using the post-ashing photoresist layer R as a mask. The semiconductor layer 800 obtained as a result of the etching has a stepped edge. The etching is performed after the gas in the device is replaced with a gas which removes the semiconductor layer S′ but does not remove the photoresist layer R.
Then, as shown in FIG. 11(e), the photoresist layer R is removed. In this manner, the semiconductor layer 800 having a stepped edge is formed.